Toroids are 30-40% lighter and more compact than E-I transformers. One reason is that the magnetic flux runs parallel to the grain orientation in all parts of the core and another that less wire is needed because the windings are distributed evenly over the entire core circumference, resulting in a shorter mean turn length.

The performance of a toroidal transformer may be further optimized with an O-core (like in O-ring). O cores are wound from a silicon steel strip, which is continuously pointed towards each end to produce a circular cross section.

They are more expensive to produce than regular toroids and inevitably involve some scrap losses when the strip is cut to shape, but on the other hand feature advantages, which sometimes are worth the extra cost.

Why should you consider the O-core:

Alpha-Core now introduces a series of O-cores for power transformers ranging from 12 to 1200VA at 60Hz. They are made of the best quality Japanese silicon steel and may be used safely up to 17kG.